Those skilled in the hydrocarbon recovery industry recognize that an increasing percentage of hydrocarbons are being recovered from offshore wells, including wells wherein the subsea wellhead is located in very deep water of 6,000 feet or more below the ocean surface. Subsea blowout preventers (BOPs) and related production control equipment rely upon a source of pressurized fluid to actuate the subsea equipment. Much of this equipment must be actuatable in at least two directions and thus is operated by supplying a hydraulic fluid pressure to either "open" or "close" the equipment. A reliable hydraulic control system to operate the equipment is particularly important in emergency applications wherein the equipment must be actuated to either the closed or the opened position in an emergency.
When subsea equipment is positioned in relatively shallow water of two thousand or three thousand feet, a reliable pressure source to operate the subsea equipment commonly is provided by a bank of accumulator bottles (accumulators), which are conventionally precharged with nitrogen. Each accumulator is thus a sealed container which houses pressurized nitrogen, and a bank of such accumulators may be fluidly interconnected to provide the power source for operating the subsea equipment. The nitrogen thus acts as an available spring force to operate the subsea equipment once hydraulic fluid under pressure is pumped into the accumulators from an external source at the surface. Once the subsea accumulators are activated, additional hydraulic fluid is conventionally transmitted from the surface to the subsea accumulators through hose ambilicals or relatively small conduit fill lines.
While the accumulator system as discussed above performs well on land operations and in relatively shallow subsea operations, significant problems are encountered using this accumulator system at water depths of more than 6,000 feet. The nitrogen precharge pressure must be increased to overcome the effects of hydrostatic head pressure for water depth of the control system. Nitrogen, like other gases, has a reduced expansion as the pressure to which it is subjected gets higher. Moreover, subsea equipment at 10,000 feet or more is inherently cool, and the combination of the cooled and high pressure nitrogen approaches saturation so that the nitrogen tends to lose its expanding characteristics and thus its pressurizing ability on the subsea equipment. As a consequence, numerous banks of accumulators are required to reliably supply activating fluid to a BOP at 10,000 feet, although the same BOP may be reliably controlled at the surface or in shallow waters with only a few accumulators.
At deep water depths, the hydraulic energy stored in the accumulators may be at a pressure of several thousand psi in addition to the hydrostatic head pressure of the surrounding sea water. At a depth of 10,000 feet, for example, the stored pressure would be approximately 5,000 psi plus 4,450 psi hydrostatic head, for a total of 9,450 psi. At this high pressure, the nitrogen in the accumulator is very inefficient since flow characteristics of the nitrogen become very sluggish. For the required capacity of fluid to reliably operate the subsea equipment, the quantity of accumulators required is thus significantly increased. This large number of accumulators represents a high cost to supply fluid to operate the subsea equipment, thereby increasing the overall cost of the hydrocarbon recovery operation.
The disadvantages of the prior art are overcome by the present invention, and an improved hydraulic control system for operating subsea equipment is hereinafter disclosed.